1. Field of the Invention
The invention relates to a device for operating a high pressure discharge lamp. The invention relates especially to a device for operating a high pressure discharge lamp which is suited for use for a light source of a projector of the projection type or the like.
2. Description of the Related Art
In a device for operating a discharge lamp, it is known to employ a circuit having a full bridge system of a low frequency of 50/60 Hz in which a high frequency of roughly 20 kHz is superimposed and operation with an alternating high frequency achieved (for example, Japanese patent disclosure HEI 6-65175).
The process of starting the known discharge lamp is as follows. Beginning in the no-load state an igniter circuit superimposes a high voltage pulse which starts a discharge lamp. Thereafter, from the current supply of the feed device of the discharge space the lamp passes from a glow discharge state into an arc discharge state to achieve a steady operating state.
The above described transition from the glow discharge state into the arc discharge state is often a so-called half wave discharge and which does not always takes place advantageously. In the above described discharge lamp of Japanese patent disclosure HEI 6-65175, in order to eliminate this disadvantage, for the duration of the transition from the glow discharge state into an arc discharge state a DC voltage or an AC voltage with a frequency which is lower than in steady-state operation is applied to the discharge lamp.
In either the situation that, at the beginning of starting, a polarity inversion of the current is not carried out, or that at the frequency at which the discharge at the beginning of starting increases sufficiently and at which the re-ignition voltage becomes low enough, an attempt is made to carry out a polarity inversion in order to accomplish advantageous starting efficiency. This is because the so-called re-ignition voltage is formed and because lamp extinction or flickering occurs when it does not proceed as far as to lamp extinction if during the glow discharge, at the beginning of starting, the polarity of the current flowing in the discharge lamp is inverted.
On the other hand, recently a high pressure discharge lamp filled with a large amount of mercury has been considered a light source of a projector device of the projection type or the like. Specifically the amount of mercury added is at least 0.15 mg/mm3 and the vapor pressure during operation is greater than or equal to 150 atm, even if it also depends on the temperature condition and the like. This discharge lamp emits light in the visible range by increasing the mercury vapor pressure, especially continuous spectrum light with an increased red portion. This discharge lamp has good color rendering and high light intensity.
The above described high pressure discharge lamp with greater than or equal to 0.15 mg/mm3 mercury added is repeatedly turned on and off according to the use of a projector device. In the above described high pressure discharge lamp, the mercury during operation is present as vapor, and when turned off, as liquid. The liquid mercury normally adheres to the electrodes with the lowest temperature. The electrodes consist of a metal such as tungsten or the like. Therefore the temperature decreases rapidly. The mercury adhering to the two electrodes is however not the same depending on the cooling state, the variance of the electrode positions and the like, but normally it adheres to one of the electrodes in a large amount.
The reason for this is the following. Due to the deviation in the positional relationship of the two electrodes to each another during the fabrication of the lamp, due to the positional relationship of the lamp to the reflector in the case of using a lamp installed in a reflector, or based on the cooling conditions and the like, a difference arises in the question of during which interval, after the lamp is turned off, does the cooling the temperatures of the two electrodes drop almost to room temperature.
If, during the glow discharge, at the beginning of the starting of the discharge lamp, a DC voltage is applied by the feed device to the above described high pressure discharge lamp as is described in Japanese patent disclosure document HEI 6-65175, a rapid transition takes place from the glow discharge state into the arc discharge state when the electrode on the side on which a large amount of mercury adheres is the cathode. However, in the reverse case, the transition to the arc discharge does not take place quickly. There are also situations in which a glow discharge arises over ten and a few ms. The glow discharge which exists over an interval of this length sputters the electrode material, causes blackening of the inside of the discharge vessel and causes a reduction of the light flux of the lamp.